Is the 6 kDa tobacco etch viral protein a bona fide ERES marker?

Age-dependent changes in leaf size in apple are governed by a cytokinin-integrated module

Plants undergo various age-dependent changes in leaf morphology during juvenile to adult vegetative stage. However, the precise molecular mechanisms governing these changes in apple (Malus domestica) remain unknown. Here, we showed that CYTOKININ OXIDASE/DEHYDROGENASE5 (MdCKX5), an age-dependent gene, encodes a functional CKX enzyme and serves as the common downstream target of SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) transcription factor MdSPL14 and WRKY transcription factor MdWRKY24 to control the degradation of cytokinin (CK). As the target of mdm-microRNA156a, MdSPL14 interacts with MdWRKY24 to coordinately repress the transcription of MdCKX5 by forming the age-mediated mdm-miR156a-MdSPL14-MdWRKY24 module, which regulates age-dependent changes in CK during the juvenile-to-adult phase transition. We further demonstrated that MdARR6, a type-A ARABIDOPSIS RESPONSE REGULATOR (ARR), is a negative feedback regulator in the CK signaling pathway. Silencing of MdARR6 in apple resulted in large leaves with smaller epidermal cells and a greater number of epidermal cells. Biochemical analysis showed that the mdm-miR156a-MdSPL14-MdWRKY24 module acts as a transcriptional repressor to directly regulate MdARR6 expression, thus controlling the age-dependent changes in leaf size by reducing CK responses. These findings established a link between the age pathway and CK signaling and revealed the molecular mechanism underlying age-dependent changes during the juvenile-to-adult phase transition; our results also provide targets for the genetic improvement of the vegetative phase transition in apple.

Distinct Mechanisms of Endomembrane Reorganization Determine Dissimilar Transport Pathways in Plant RNA Viruses

Plant viruses exploit the endomembrane system of infected cells for their replication and cell-to-cell transport. The replication of viral RNA genomes occurs in the cytoplasm in association with reorganized endomembrane compartments induced by virus-encoded proteins and is coupled with the virus intercellular transport via plasmodesmata that connect neighboring cells in plant tissues. The transport of virus genomes to and through plasmodesmata requires virus-encoded movement proteins (MPs). Distantly related plant viruses encode different MP sets, or virus transport systems, which vary in the number of MPs and their properties, suggesting their functional differences. Here, we discuss two distinct virus transport pathways based on either the modification of the endoplasmic reticulum tubules or the formation of motile vesicles detached from the endoplasmic reticulum and targeted to endosomes. The viruses with the movement proteins encoded by the triple gene block exemplify the first, and the potyviral system is the example of the second type. These transport systems use unrelated mechanisms of endomembrane reorganization. We emphasize that the mode of virus interaction with cell endomembranes determines the mechanism of plant virus cell-to-cell transport.

Characterization of Arabidopsis Post-Glycosylphosphatidylinositol Attachment to Proteins Phospholipase 3 Like Genes

Lipid remodeling of Glycosylphosphatidylinositol (GPI) anchors is required for their maturation and may influence the localization and function of GPI-anchored proteins (GPI-APs). Maturation of GPI-anchors is well characterized in animals and fungi but very little is known about this process in plants. In yeast, the GPI-lipid remodeling occurs entirely at the ER and is initiated by the remodeling enzyme Bst1p (Post-Glycosylphosphatidylinositol Attachment to Proteins inositol deacylase 1 -PGAP1- in mammals and Arabidopsis). Next, the remodeling enzyme Per1p (Post-Glycosylphosphatidylinositol Attachment to Proteins phospholipase 3 -PGAP3- in mammals) removes a short, unsaturated fatty acid of phosphatidylinositol (PI) that is replaced with a very long-chain saturated fatty acid or ceramide to complete lipid remodeling. In mammals, lipid remodeling starts at the ER and is completed at the Golgi apparatus. Studies of the Arabidopsis PGAP1 gene showed that the lipid remodeling of the GPI anchor is critical for the final localization of GPI-APs. Here we characterized loss-of-function mutants of Arabidopsis Per1/PGAP3 like genes (AtPGAP3A and AtPGAP3B). Our results suggest that PGAP3A function is required for the efficient transport of GPI-anchored proteins from the ER to the plasma membrane/cell wall. In addition, loss of function of PGAP3A increases susceptibility to salt and osmotic stresses that may be due to the altered localization of GPI-APs in this mutant. Furthermore, PGAP3B complements a yeast strain lacking PER1 gene suggesting that PGAP3B and Per1p are functional orthologs. Finally, subcellular localization studies suggest that PGAP3A and PGAP3B cycle between the ER and the Golgi apparatus.

AtPGAP1 functions as a GPI inositol-deacylase required for efficient transport of GPI-anchored proteins

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) play an important role in a variety of plant biological processes including growth, stress response, morphogenesis, signaling, and cell wall biosynthesis. The GPI anchor contains a lipid-linked glycan backbone that is synthesized in the endoplasmic reticulum (ER) where it is subsequently transferred to the C-terminus of proteins containing a GPI signal peptide by a GPI transamidase. Once the GPI anchor is attached to the protein, the glycan and lipid moieties are remodeled. In mammals and yeast, this remodeling is required for GPI-APs to be included in Coat Protein II-coated vesicles for their ER export and subsequent transport to the cell surface. The first reaction of lipid remodeling is the removal of the acyl chain from the inositol group by Bst1p (yeast) and Post-GPI Attachment to Proteins Inositol Deacylase 1 (PGAP1, mammals). In this work, we have used a loss-of-function approach to study the role of PGAP1/Bst1 like genes in plants. We have found that Arabidopsis (Arabidopsis thaliana) PGAP1 localizes to the ER and likely functions as the GPI inositol-deacylase that cleaves the acyl chain from the inositol ring of the GPI anchor. In addition, we show that PGAP1 function is required for efficient ER export and transport to the cell surface of GPI-APs.

Arabidopsis HIPP proteins regulate endoplasmic reticulum-associated degradation of CKX proteins and cytokinin responses

Eukaryotic organisms are equipped with quality-control mechanisms that survey protein folding in the endoplasmic reticulum (ER) and remove non-native proteins by ER-associated degradation (ERAD). Recent research has shown that cytokinin-degrading CKX proteins are subjected to ERAD during plant development. The mechanisms of plant ERAD, including the export of substrate proteins from the ER, are not fully understood, and the molecular components involved in the ERAD of CKX are unknown. Here, we show that heavy metal-associated isoprenylated plant proteins (HIPPs) interact specifically with CKX proteins synthesized in the ER and processed by ERAD. CKX-HIPP protein complexes were detected at the ER as well as in the cytosol, suggesting that the complexes involve retrotranslocated CKX protein species. Altered CKX levels in HIPP-overexpressing and higher-order hipp mutant plants suggest that the studied HIPPs control the ERAD of CKX. Deregulation of CKX proteins caused corresponding changes in the cytokinin signaling activity and triggered typical morphological cytokinin responses. Notably, transcriptional repression of HIPP genes by cytokinin indicates a feedback regulatory mechanism of cytokinin homeostasis and signaling responses. Moreover, loss of function of HIPP genes constitutively activates the unfolded protein response and compromises the ER stress tolerance. Collectively, these results suggests that HIPPs represent novel functional components of plant ERAD.

Pepper Mottle Virus and Its Host Interactions: Current State of Knowledge

Pepper mottle virus (PepMoV) is a destructive pathogen that infects various solanaceous plants, including pepper, bell pepper, potato, and tomato. In this review, we summarize what is known about the molecular characteristics of PepMoV and its interactions with host plants. Comparisons of symptom variations caused by PepMoV isolates in plant hosts indicates a possible relationship between symptom development and genetic variation. Researchers have investigated the PepMoV–plant pathosystem to identify effective and durable genes that confer resistance to the pathogen. As a result, several recessive pvr or dominant Pvr resistance genes that confer resistance to PepMoV in pepper have been characterized. On the other hand, the molecular mechanisms underlying the interaction between these resistance genes and PepMoV-encoded genes remain largely unknown. Our understanding of the molecular interactions between PepMoV and host plants should be increased by reverse genetic approaches and comprehensive transcriptomic analyses of both the virus and the host genes.

A systematic review of Sec24 cargo interactome

Endoplasmic reticulum (ER)-to-Golgi trafficking is an essential and highly conserved cellular process. The coat protein complex-II (COPII) arm of the trafficking machinery incorporates a wide array of cargo proteins into vesicles through direct or indirect interactions with Sec24, the principal subunit of the COPII coat. Approximately one-third of all mammalian proteins rely on the COPII-mediated secretory pathway for membrane insertion or secretion. There are four mammalian Sec24 paralogs and three yeast Sec24 paralogs with emerging evidence of paralog-specific cargo interaction motifs. Furthermore, individual paralogs also differ in their affinity for a subset of sorting motifs present on cargo proteins. As with many aspects of protein trafficking, we lack a systematic and thorough understanding of the interaction of Sec24 with cargoes. This systematic review focuses on the current knowledge of cargo binding to both yeast and mammalian Sec24 paralogs and their ER export motifs. The analyses show that Sec24 paralog specificity of cargo (and cargo receptors) range from exclusive paralog dependence or preference to partial redundancy. We also discuss how the Sec24 secretion system is hijacked by viral (eg, VSV-G, Hepatitis B envelope protein) and bacterial (eg, the enteropathogenic Escherichia coli type III secretion system effector NleA/EspI) pathogens.

Comparative anatomical and transcriptomic analyses of the color variation of leaves in Aquilaria sinensis

Color variation in plant tissues is a common phenomenon accompanied with a series of biological changes. In this study, a special-phenotype Aquilaria sinensis (GS) with color variation of leaf was firstly reported, and DNA barcode sequences showed GS samples could not be discriminated clearly with the normal A. sinensis sample (NS), which suggested that the variety was not the cause of the GS formation. To reveal the characteristics of GS compared to NS, the anatomical and transcriptome sequencing studies were carried out. In microscopic observation, the leaves of golden-vein-leaf sample (LGS) and normal-vein-leaf sample (LNS) showed significant differences including the area of the included phloem in midrib and the thickness parameters of palisade and spongy tissues; the stems of golden-vein-leaf sample (SGS) and normal-vein-leaf sample (SNS) were also different in many aspects such as the area of vessels and included phloem. In addition, the structure of chloroplast was more complete in the midrib of LNS than that of LGS, and some particles suspected as virus were found through transmission electron microscope as well. Genes upregulated in LGS in contrast with LNS were mainly enriched in photosynthesis. As for stems, most of the genes upregulated in SGS compared to SNS were involved in translation and metabolism processes. The pathways about photosynthesis and chlorophyll metabolism as well as some important transcription factors may explain the molecular mechanism of the unique phenotypes of leaves and the genes related to suberin biosynthesis may result in the difference of stems. In addition, the genes about defense response especially biotic stress associated with numerous pathogenesis-related (PR) genes upregulated in LGS compared to LNS indicated that the pathogen may be the internal factor. Taken together, our results reveal the macro- and micro-phenotype variations as well as gene expression profiles between GS and NS, which could provide valuable clues for elucidating the mechanism of the color variation of Aquilaria.

Transient Expression of Fluorescent Fusion Proteins in Arabidopsis Protoplasts

Transient expression using protoplasts isolated from Arabidopsis suspension culture cells is a fast and useful tool for analyzing protein subcellular localization and dynamics in plant cells. Recently, super-resolution imaging techniques such as N-SIM (Nikon, Structured Illumination Microscopy) are widely used in cell biology study, allowing cell biologists to obtain unattainable details and relationships of cell structures and functions by conventional confocal imaging. To facilitate the usage of protoplasts transient expression and super-resolution imaging for protein localization and dynamic analysis in plant cell biology research, here we describe updated protocols of protoplasts isolation from Arabidopsis suspension culture cells and transient expression assay for protein trafficking and localization study. Further, using GFP-tagged ERES (Endoplasmic Reticulum Exit Site) marker proteins and RFP-tagged Golgi marker as examples, we illustrate the major tools and methods for protein localization analysis using super-resolution imaging.

p24 Family Proteins Are Involved in Transport to the Plasma Membrane of GPI-Anchored Proteins in Plants

p24 proteins are a family of type-I membrane proteins that cycle between the endoplasmic reticulum (ER) and the Golgi apparatus via Coat Protein I (COPI)- and COPII-coated vesicles. These proteins have been proposed to function as cargo receptors, but the identity of putative cargos in plants is still elusive. We previously generated an Arabidopsis (Arabidopsis thaliana) quadruple loss-of-function mutant affecting p24 genes from the δ-1 subclass of the p24 delta subfamily (p24δ3δ4δ5δ6 mutant). This mutant also had reduced protein levels of other p24 family proteins and was found to be sensitive to salt stress. Here, we used this mutant to test the possible involvement of p24 proteins in the transport to the plasma membrane of glycosylphosphatidylinositol (GPI)-anchored proteins. We found that GPI-anchored proteins mostly localized to the ER in p24δ3δ4δ5δ6 mutant cells, in contrast to plasma membrane proteins with other types of membrane attachment. The plasma membrane localization of GPI-anchored proteins was restored in the p24δ3δ4δ5δ6 mutant upon transient expression of a single member of the p24 δ-1 subclass, RFP-p24δ5, which was dependent on the coiled-coil domain in p24δ5. The coiled-coil domain was also important for a direct interaction between p24δ5 and the GPI-anchored protein arabinogalactan protein4 (AGP4). These results suggest that Arabidopsis p24 proteins are involved in ER export and transport to the plasma membrane of GPI-anchored proteins.

Small hydrophobic viral proteins involved in intercellular movement of diverse plant virus genomes

Most plant viruses code for movement proteins (MPs) targeting plasmodesmata to enable cell-to-cell and systemic spread in infected plants. Small membrane-embedded MPs have been first identified in two viral transport gene modules, triple gene block (TGB) coding for an RNA-binding helicase TGB1 and two small hydrophobic proteins TGB2 and TGB3 and double gene block (DGB) encoding two small polypeptides representing an RNA-binding protein and a membrane protein. These findings indicated that movement gene modules composed of two or more cistrons may encode the nucleic acid-binding protein and at least one membrane-bound movement protein. The same rule was revealed for small DNA-containing plant viruses, namely, viruses belonging to genus Mastrevirus (family Geminiviridae) and the family Nanoviridae. In multi-component transport modules the nucleic acid-binding MP can be viral capsid protein(s), as in RNA-containing viruses of the families Closteroviridae and Potyviridae. However, membrane proteins are always found among MPs of these multicomponent viral transport systems. Moreover, it was found that small membrane MPs encoded by many viruses can be involved in coupling viral replication and cell-to-cell movement. Currently, the studies of evolutionary origin and functioning of small membrane MPs is regarded as an important pre-requisite for understanding of the evolution of the existing plant virus transport systems. This paper represents the first comprehensive review which describes the whole diversity of small membrane MPs and presents the current views on their role in plant virus movement.

Mutagenesis Scanning Uncovers Evolutionary Constraints on Tobacco Etch Potyvirus Membrane-Associated 6K2 Protein

RNA virus high mutation rate is a double-edged sword. At the one side, most mutations jeopardize proteins functions; at the other side, mutations are needed to fuel adaptation. The relevant question then is the ratio between beneficial and deleterious mutations. To evaluate this ratio, we created a mutant library of the 6K2 gene of tobacco etch potyvirus that contains every possible single-nucleotide substitution. 6K2 protein anchors the virus replication complex to the network of endoplasmic reticulum membranes. The library was inoculated into the natural host Nicotiana tabacum, allowing competition among all these mutants and selection of those that are potentially viable. We identified 11 nonsynonymous mutations that remain in the viral population at measurable frequencies and evaluated their fitness. Some had fitness values higher than the wild-type and some were deleterious. The effect of these mutations in the structure, transmembrane properties, and function of 6K2 was evaluated in silico. In parallel, the effect of these mutations in infectivity, virus accumulation, symptoms development, and subcellular localization was evaluated in the natural host. The α-helix H1 in the N-terminal part of 6K2 turned out to be under purifying selection, while most observed mutations affect the link between transmembrane α-helices H2 and H3, fusing them into a longer helix and increasing its rigidity. In general, these changes are associated with higher within-host fitness and development of milder or no symptoms. This finding suggests that in nature selection upon 6K2 may result from a tradeoff between within-host accumulation and severity of symptoms.

RNA on the move: The plasmodesmata perspective

It is now widely accepted that plant RNAs can have effects at sites far away from their sites of synthesis. Cellular mRNA transcripts, endogenous small RNAs and defense-related small RNAs all move from cell to cell via plasmodesmata (PD), and may even move long distances in the phloem. Despite their small size, PD have complicated substructures, and the area of the pore available for RNA trafficking can be remarkably small. The intent of this review is to bring into focus the role of PD in cell-to-cell and long distance communication in plants. We consider how cellular RNAs could move through the cell to the PD and thence through PD. The protein composition of PD and the possible roles of PD proteins in RNA trafficking are also discussed. Recent evidence for RNA metabolism in organelles acting as a factor in controlling PD flux is also presented, highlighting new aspects of plant intra- and intercellular communication. It is clear that while the phenomenon of RNA mobility is common and essential, many questions remain, and these have been highlighted throughout this review.

A Functional Link between RNA Replication and Virion Assembly in the Potyvirus Plum Pox Virus

Accurate assembly of viral particles in the potyvirus Plum pox virus (PPV) has been shown to depend on the contribution of the multifunctional viral protein HCPro. In this study, we show that other viral factors, in addition to the capsid protein (CP) and HCPro, are necessary for the formation of stable PPV virions. The CP produced in Nicotiana benthamiana leaves from a subviral RNA termed LONG, which expresses a truncated polyprotein that lacks P1 and HCPro, together with HCPro supplied in trans, was assembled into virus-like particles and remained stable after in vitro incubation. In contrast, deletions in multiple regions of the LONG coding sequence prevented the CP stabilization mediated by HCPro. In particular, we demonstrated that the first 178 amino acids of P3, but not a specific nucleotide sequence coding for them, are required for CP stability and proper assembly of PPV particles. Using a sequential coagroinfiltration assay, we observed that the subviral LONG RNA replicates and locally spreads in N. benthamiana leaves expressing an RNA silencing suppressor. The analysis of the effect of both point and deletion mutations affecting RNA replication in LONG and full-length PPV demonstrated that this process is essential for the assembly of stable viral particles. Interestingly, in spite of this requirement, the CP produced by a nonreplicating viral RNA can be stably assembled into virions as long as it is coexpressed with a replication-proficient RNA. Altogether, these results highlight the importance of coupling encapsidation to other viral processes to secure a successful infection.IMPORTANCE Viruses of the family Potyviridae are among the most dangerous threats for basically every important crop, and such socioeconomical relevance has made them a subject of many research studies. In spite of this, very little is currently known about proteins and processes controlling viral genome encapsidation by the coat protein. In the case of Plum pox virus (genus Potyvirus), for instance, we have previously shown that the multitasking viral factor HCPro plays a role in the production of stable virions. Here, by using this potyvirus as a model, we move further to show that additional factors are also necessary for the efficient production of potyviral particles. More importantly, a comprehensive screening for such factors led us to the identification of a functional link between virus replication and packaging, unraveling a previously unknown connection of these two key events of the potyviral infection cycle.

The Cytokinin Oxidase/Dehydrogenase CKX1 Is a Membrane-Bound Protein Requiring Homooligomerization in the Endoplasmic Reticulum for Its Cellular Activity

Degradation of the plant hormone cytokinin is controlled by cytokinin oxidase/dehydrogenase (CKX) enzymes. The molecular and cellular behavior of these proteins is still largely unknown. In this study, we show that CKX1 is a type II single-pass membrane protein that localizes predominantly to the endoplasmic reticulum (ER) in Arabidopsis (Arabidopsis thaliana). This indicates that this CKX isoform is a bona fide ER protein directly controlling the cytokinin, which triggers the signaling from the ER. By using various approaches, we demonstrate that CKX1 forms homodimers and homooligomers in vivo. The amino-terminal part of CKX1 was necessary and sufficient for the protein oligomerization as well as for targeting and retention in the ER. Moreover, we show that protein-protein interaction is largely facilitated by transmembrane helices and depends on a functional GxxxG-like interaction motif. Importantly, mutations rendering CKX1 monomeric interfere with its steady-state localization in the ER and cause a loss of the CKX1 biological activity by increasing its ER-associated degradation. Therefore, our study provides evidence that oligomerization is a crucial parameter regulating CKX1 biological activity and the cytokinin concentration in the ER. The work also lends strong support for the cytokinin signaling from the ER and for the functional relevance of the cytokinin pool in this compartment.

COPII Paralogs in Plants: Functional Redundancy or Diversity?

In eukaryotes, the best-described mechanism of endoplasmic reticulum (ER) export is mediated by coat protein complex II (COPII) vesicles, which comprise five conserved cytosolic components [secretion-associated, Ras-related protein 1 (Sar1), Sec23-24, and Sec13-31]. In higher organisms, multiple paralogs of COPII components are created due to gene duplication. However, the functional diversity of plant COPII subunit isoforms remains largely elusive. Here we summarize and discuss the latest findings derived from studies of various arabidopsis COPII subunit isoforms and their functional diversity. We also put forward testable hypotheses on distinct populations of COPII vesicles performing unique functions in ER export in developmental and stress-related pathways in plants.

Protein composition of 6K2-induced membrane structures formed during Potato virus A infection

The definition of the precise molecular composition of membranous replication compartments is a key to understanding the mechanisms of virus multiplication. Here, we set out to investigate the protein composition of the potyviral replication complexes. We purified the potyviral 6K2 protein-induced membranous structures from Potato virus A (PVA)-infected Nicotiana benthamiana plants. For this purpose, the 6K2 protein, which is the main inducer of potyviral membrane rearrangements, was expressed in fusion with an N-terminal Twin-Strep-tag and Cerulean fluorescent protein (SC6K) from the infectious PVA cDNA. A non-tagged Cerulean-6K2 (C6K) virus and the SC6K protein alone in the absence of infection were used as controls. A purification scheme exploiting discontinuous sucrose gradient centrifugation followed by Strep-tag-based affinity chromatography was developed. Both (+)- and (-)-strand PVA RNA and viral protein VPg were co-purified specifically with the affinity tagged PVA-SC6K. The purified samples, which contained individual vesicles and membrane clusters, were subjected to mass spectrometry analysis. Data analysis revealed that many of the detected viral and host proteins were either significantly enriched or fully specifically present in PVA-SC6K samples when compared with the controls. Eight of eleven potyviral proteins were identified with high confidence from the purified membrane structures formed during PVA infection. Ribosomal proteins were identified from the 6K2-induced membranes only in the presence of a replicating virus, reinforcing the tight coupling between replication and translation. A substantial number of proteins associating with chloroplasts and several host proteins previously linked with potyvirus replication complexes were co-purified with PVA-derived SC6K, supporting the conclusion that the host proteins identified in this study may have relevance in PVA replication.

The Vesicle-Forming 6K2 Protein of Turnip Mosaic Virus Interacts with the COPII Coatomer Sec24a for Viral Systemic Infection

Positive-sense RNA viruses remodel host cell endomembranes to generate quasi-organelles known as "viral factories" to coordinate diverse viral processes, such as genome translation and replication. It is also becoming clear that enclosing viral RNA (vRNA) complexes within membranous structures is important for virus cell-to-cell spread throughout the host. In plant cells infected by turnip mosaic virus (TuMV), a member of the family Potyviridae, peripheral motile endoplasmic reticulum (ER)-derived viral vesicles are produced that carry the vRNA to plasmodesmata for delivery into adjacent noninfected cells. The viral protein 6K2 is responsible for the formation of these vesicles, but how 6K2 is involved in their biogenesis is unknown. We show here that 6K2 is associated with cellular membranes. Deletion mapping and site-directed mutagenesis experiments defined a soluble N-terminal 12-amino-acid stretch, in particular a potyviral highly conserved tryptophan residue and two lysine residues that were important for vesicle formation. When the tryptophan residue was changed into an alanine in the viral polyprotein, virus replication still took place, albeit at a reduced level, but cell-to-cell movement of the virus was abolished. Yeast (Saccharomyces cerevisiae) two-hybrid and coimmunoprecipitation experiments showed that 6K2 interacted with Sec24a, a COPII coatomer component. Appropriately, TuMV systemic movement was delayed in an Arabidopsis thaliana mutant line defective in Sec24a. Intercellular movement of TuMV replication vesicles thus requires ER export of 6K2, which is mediated by the interaction of the N-terminal domain of the viral protein with Sec24a.

IMPORTANCE

Many plant viruses remodel the endoplasmic reticulum (ER) to generate vesicles that are associated with the virus replication complex. The viral protein 6K2 of turnip mosaic virus (TuMV) is known to induce ER-derived vesicles that contain vRNA as well as viral and host proteins required for vRNA synthesis. These vesicles not only sustain vRNA synthesis, they are also involved in the intercellular trafficking of vRNA. In this investigation, we found that the N-terminal soluble domain of 6K2 is required for ER export of the protein and for the formation of vesicles. ER export is not absolutely required for vRNA replication but is necessary for virus cell-to-cell movement. Furthermore, we found that 6K2 physically interacts with the COPII coatomer Sec24a and that an Arabidopsis thaliana mutant line with a defective Sec24a shows a delay in the systemic infection by TuMV.

The Vesicle-Forming 6K2 Protein of Turnip Mosaic Virus Interacts with the COPII Coatomer Sec24a for Viral Systemic Infection

Positive-sense RNA viruses remodel host cell endomembranes to generate quasi-organelles known as "viral factories" to coordinate diverse viral processes, such as genome translation and replication. It is also becoming clear that enclosing viral RNA (vRNA) complexes within membranous structures is important for virus cell-to-cell spread throughout the host. In plant cells infected by turnip mosaic virus (TuMV), a member of the family Potyviridae, peripheral motile endoplasmic reticulum (ER)-derived viral vesicles are produced that carry the vRNA to plasmodesmata for delivery into adjacent noninfected cells. The viral protein 6K2 is responsible for the formation of these vesicles, but how 6K2 is involved in their biogenesis is unknown. We show here that 6K2 is associated with cellular membranes. Deletion mapping and site-directed mutagenesis experiments defined a soluble N-terminal 12-amino-acid stretch, in particular a potyviral highly conserved tryptophan residue and two lysine residues that were important for vesicle formation. When the tryptophan residue was changed into an alanine in the viral polyprotein, virus replication still took place, albeit at a reduced level, but cell-to-cell movement of the virus was abolished. Yeast (Saccharomyces cerevisiae) two-hybrid and coimmunoprecipitation experiments showed that 6K2 interacted with Sec24a, a COPII coatomer component. Appropriately, TuMV systemic movement was delayed in an Arabidopsis thaliana mutant line defective in Sec24a. Intercellular movement of TuMV replication vesicles thus requires ER export of 6K2, which is mediated by the interaction of the N-terminal domain of the viral protein with Sec24a.

IMPORTANCE

Many plant viruses remodel the endoplasmic reticulum (ER) to generate vesicles that are associated with the virus replication complex. The viral protein 6K2 of turnip mosaic virus (TuMV) is known to induce ER-derived vesicles that contain vRNA as well as viral and host proteins required for vRNA synthesis. These vesicles not only sustain vRNA synthesis, they are also involved in the intercellular trafficking of vRNA. In this investigation, we found that the N-terminal soluble domain of 6K2 is required for ER export of the protein and for the formation of vesicles. ER export is not absolutely required for vRNA replication but is necessary for virus cell-to-cell movement. Furthermore, we found that 6K2 physically interacts with the COPII coatomer Sec24a and that an Arabidopsis thaliana mutant line with a defective Sec24a shows a delay in the systemic infection by TuMV.

Molecular biology of potyviruses

Potyvirus is the largest genus of plant viruses causing significant losses in a wide range of crops. Potyviruses are aphid transmitted in a nonpersistent manner and some of them are also seed transmitted. As important pathogens, potyviruses are much more studied than other plant viruses belonging to other genera and their study covers many aspects of plant virology, such as functional characterization of viral proteins, molecular interaction with hosts and vectors, structure, taxonomy, evolution, epidemiology, and diagnosis. Biotechnological applications of potyviruses are also being explored. During this last decade, substantial advances have been made in the understanding of the molecular biology of these viruses and the functions of their various proteins. After a general presentation on the family Potyviridae and the potyviral proteins, we present an update of the knowledge on potyvirus multiplication, movement, and transmission and on potyvirus/plant compatible interactions including pathogenicity and symptom determinants. We end the review providing information on biotechnological applications of potyviruses.

Arabidopsis p24δ5 and p24δ9 facilitate Coat Protein I-dependent transport of the K/HDEL receptor ERD2 from the Golgi to the endoplasmic reticulum

The p24 proteins belong to a family of type I membrane proteins which cycle between the endoplasmic reticulum (ER) and Golgi via coat protein I (COPI) and COPII vesicles. Current nomenclature classifies them into four subfamilies, although plant p24 proteins belong to either the p24β or the p24δ subfamilies. Here, we show that Arabidopsis p24δ5/δ9 and HDEL ligands shift the steady-state distribution of the K/HDEL receptor ERD2 from the Golgi to the ER. We also show that p24δ5/δ9 interact directly with ERD2. This interaction requires the Golgi dynamics (GOLD) domain in p24δ5 and is much higher at acidic than at neutral pH, consistent with both proteins interacting at the cis-Golgi. In addition, p24δ5 also inhibits the secretion of HDEL ligands, but not constitutive secretion, showing a role for p24δ5 in retrograde Golgi-to-ER transport. Both p24δ5 and ERD2 interact with ADP-ribosylation factor 1 (ARF1) and COPI subunits, mostly at acidic pH, consistent with COPI vesicles being involved in retrograde transport of both proteins. In contrast, both proteins interact with the COPII subunit Sec23, mostly at neutral pH, consistent with this interaction taking place at the ER for anterograde transport to the Golgi apparatus.

Traffic into silence: endomembranes and post-transcriptional RNA silencing

microRNAs (miRNAs) and small interfering RNAs (siRNAs) are small RNAs that repress gene expression at the post-transcriptional level in plants and animals. Small RNAs guide Argonaute-containing RNA-induced silencing complexes to target RNAs in a sequence-specific manner, resulting in mRNA deadenylation followed by exonucleolytic decay, mRNA endonucleolytic cleavage, or translational inhibition. Although our knowledge of small RNA biogenesis, turnover, and mechanisms of action has dramatically expanded in the past decade, the subcellular location of small RNA-mediated RNA silencing still needs to be defined. In contrast to the prevalent presumption that RNA silencing occurs in the cytosol, emerging evidence reveals connections between the endomembrane system and small RNA activities in plants and animals. Here, we summarize the work that uncovered this link between small RNAs and endomembrane compartments and present an overview of the involvement of the endomembrane system in various aspects of RNA silencing. We propose that the endomembrane system is an integral component of RNA silencing that has been long overlooked and predict that a marriage between cell biology and RNA biology holds the key to a full understanding of post-transcriptional gene regulation by small RNAs.

Putative p24 complexes in Arabidopsis contain members of the delta and beta subfamilies and cycle in the early secretory pathway

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24α, p24β, p24γ, and p24δ. In contrast to animals and fungi, plants contain only members of the p24β and p24δ subfamilies, the latter probably including two different subclasses. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24δ5 (p24δ1 subclass) localizes to the endoplasmic reticulum (ER) at steady state as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. It is now shown that transiently expressed RFP-p24δ9 (p24δ2 subclass) also localizes to the ER. In contrast, transiently expressed green fluorescent protein (GFP)-p24β3 mainly localizes to the Golgi apparatus (as p24β2) and exits the ER in a COPII-dependent manner. Immunogold electron microscopy in Arabidopsis root tip cells using specific antibodies shows that endogenous p24δ9 localizes mainly to the ER but also partially to the cis-Golgi. In contrast, endogenous p24β3 mainly localizes to the Golgi apparatus. By a combination of experiments using transient expression, knock-out mutants, and co-immunoprecipitation, it is proposed that Arabidopsis p24 proteins form different heteromeric complexes (including members of the β and δ subfamilies) which are important for their stability and their coupled trafficking at the ER-Golgi interface. Evidence is also provided for a role for p24δ5 in retrograde Golgi-ER transport of the KDEL-receptor ERD2.

Host endomembrane recruitment for plant RNA virus replication

Although there is a significant amount of literature that deals with the identification of plant viral proteins involved in membrane remodeling and vesicle production in infected cells, there are very few investigations that report on the impact that infection has on the overall architecture and dynamics of the early secretory endomembranes. Recent investigations have shown that for some viruses the endoplasmic reticulum, Golgi bodies and other organelles are heavily recruited into virus-induced perinuclear structures. These structures are not isolated organelles and are dynamically connected to the bulk of non-modified endomembranes. They also have a functional link with peripheral motile vesicles involved in virus intracellular movement. The full molecular events that consubstantiate with this endomembrane recruitment in virus-induced structures remain to be elucidated but viral genome replication and virion assembly are probably taking place within these structures.

Impact on the endoplasmic reticulum and Golgi apparatus of turnip mosaic virus infection

The impact of turnip mosaic virus (TuMV) infection on the endomembranes of the host early secretory pathway was investigated using an infectious clone that has been engineered for tagging viral membrane structures with a fluorescent protein fused to the viral protein 6K(2). TuMV infection led to the amalgamation of the endoplasmic reticulum (ER), Golgi apparatus, COPII coatamers, and chloroplasts into a perinuclear globular structure that also contained viral proteins. One consequence of TuMV infection was that protein secretion was blocked at the ER-Golgi interface. Fluorescence recovery after photobleaching (FRAP) experiments indicated that the perinuclear structure cannot be restocked in viral components but was dynamically connected to the bulk of the Golgi apparatus and the ER. Experiments with 6K(2) fused to photoactivable green fluorescent protein (GFP) showed that production of motile peripheral 6K(2) vesicles was functionally linked to the perinuclear structure. Disruption of the early secretory pathway did not prevent the formation of the perinuclear globular structure, enhanced the clustering of peripheral 6K(2) vesicles with COPII coatamers, and led to inhibition of cell-to-cell virus movement. This suggests that a functional secretory pathway is not required for the formation of the TuMV perinuclear globular structure and peripheral vesicles but is needed for successful viral intercellular propagation.

ERES (ER exit sites) and the "secretory unit concept"

The higher plant Golgi apparatus consists of hundreds of individual Golgi stacks which move along the cortical ER, propelled by the actomysin system. Anterograde and retrograde transport between the endoplasmic reticulum (ER) and the plant Golgi occurs over a narrow interface (around 500 nm) and is generally considered to be mediated by COP-coated vesicles. Previously, ER exit sites (ERES) have been identified on the basis of to localization of transiently expressed COPII-coat proteins. As a consequence it has been held that ERES in higher plants are intimately associated with Golgi stacks, and that both move together as an integrated structure: the "secretory unit". Using a new COPII marker, as well as YFP-SEC24 (a bona fide COPII coat protein), we have made observations on tobacco leaf epidermis at high resolution in the CLSM. Our data clearly shows that COPII fluorescence is associated with the Golgi stacks rather than the surface of the ER and probably represents the temporary accumulation of COPII vesicles in the Golgi matrix prior to fusion with the cis-Golgi cisternae. We have calculated the numbers of COPII vesicles which would be required to provide a typical Golgi-associated COPII-fluorescent signal as being much less than 20. We have discussed the consequences of this and question the continued usage of the term "secretory unit".

Coupled transport of Arabidopsis p24 proteins at the ER-Golgi interface

p24 proteins are a family of type I membrane proteins localized to compartments of the early secretory pathway and to coat protein I (COPI)- and COPII-coated vesicles. They can be classified, by sequence homology, into four subfamilies, named p24α, p24β, p24γ, and p24δ. In contrast to animals and fungi, plants contain only members of the p24β and p24δ subfamilies. It has previously been shown that transiently expressed red fluorescent protein (RFP)-p24δ5 localizes to the endoplasmic reticulum (ER) as a consequence of highly efficient COPI-based recycling from the Golgi apparatus. Using specific antibodies, endogenous p24δ5 has now been localized to the ER and p24β2 to the Golgi apparatus in Arabidopsis root tip cells by immunogold electron microscopy. The relative contributions of the cytosolic tail and the luminal domains to p24δ5 trafficking have also been characterized. It is demonstrated that whereas the dilysine motif in the cytoplasmic tail determines the location of p24δ5 in the early secretory pathway, the luminal domain may contribute to its distribution downstream of the Golgi apparatus. By using knock-out mutants and co-immunoprecipitation experiments, it is shown that p24δ5 and p24β2 interact with each other. Finally, it is shown that p24δ5 and p24β2 exhibit coupled trafficking at the ER-Golgi interface. It is proposed that p24δ5 and p24β2 interact with each other at ER export sites for ER exit and coupled transport to the Golgi apparatus. Once in the Golgi, p24δ5 interacts very efficiently with the COPI machinery for retrograde transport back to the ER.

Arabidopsis senescence-associated protein DMP1 is involved in membrane remodeling of the ER and tonoplast

BACKGROUND

Arabidopsis DMP1 was discovered in a genome-wide screen for senescence-associated membrane proteins. DMP1 is a member of a novel plant-specific membrane protein family of unknown function. In rosette leaves DMP1 expression increases from very low background level several 100fold during senescence progression.

RESULTS

Expression of AtDMP1 fused to eGFP in Nicotiana benthamiana triggers a complex process of succeeding membrane remodeling events affecting the structure of the endoplasmic reticulum (ER) and the vacuole. Induction of spherical structures ("bulbs"), changes in the architecture of the ER from tubular to cisternal elements, expansion of smooth ER, formation of crystalloid ER, and emergence of vacuolar membrane sheets and foamy membrane structures inside the vacuole are proceeding in this order. In some cells it can be observed that the process culminates in cell death after breakdown of the entire ER network and the vacuole. The integrity of the plasma membrane, nucleus and Golgi vesicles are retained until this stage. In Arabidopsis thaliana plants expressing AtDMP1-eGFP by the 35S promoter massive ER and vacuole vesiculation is observed during the latest steps of leaf senescence, whereas earlier in development ER and vacuole morphology are not perturbed. Expression by the native DMP1 promoter visualizes formation of aggregates termed "boluses" in the ER membranes and vesiculation of the entire ER network, which precedes disintegration of the central vacuole during the latest stage of senescence in siliques, rosette and cauline leaves and in darkened rosette leaves. In roots tips, DMP1 is strongly expressed in the cortex undergoing vacuole biogenesis.

CONCLUSIONS

Our data suggest that DMP1 is directly or indirectly involved in membrane fission during breakdown of the ER and the tonoplast during leaf senescence and in membrane fusion during vacuole biogenesis in roots. We propose that these properties of DMP1, exacerbated by transient overexpression, may cause or contribute to the dramatic membrane remodeling events which lead to cell death in infiltrated tobacco leaves.

Cytoplasmic Arabidopsis AGO7 accumulates in membrane-associated siRNA bodies and is required for ta-siRNA biogenesis

Formation of trans-acting small interfering RNAs (ta-siRNAs) from the TAS3 precursor is triggered by the AGO7/miR390 complex, which primes TAS3 for conversion into double-stranded RNA by the RNA-dependent RNA polymerase RDR6 and SGS3. These ta-siRNAs control several aspects of plant development. The mechanism routing AGO7-cleaved TAS3 precursor to RDR6/SGS3 and its subcellular organization are unknown. We show that AGO7 accumulates together with SGS3 and RDR6 in cytoplasmic siRNA bodies that are distinct from P-bodies. siRNA bodies colocalize with a membrane-associated viral protein and become positive for stress-granule markers upon stress-induced translational repression, this suggests that siRNA bodies are membrane-associated sites of accumulation of mRNA stalled during translation. AGO7 congregates with miR390 and SGS3 in membranes and its targeting to the nucleus prevents its accumulation in siRNA bodies and ta-siRNA formation. AGO7 is therefore required in the cytoplasm and membranous siRNA bodies for TAS3 processing, revealing a hitherto unknown role for membrane-associated ribonucleoparticles in ta-siRNA biogenesis and AGO action in plants.

ER Import Sites and Their Relationship to ER Exit Sites: A New Model for Bidirectional ER-Golgi Transport in Higher Plants

Per definition, ER exit sites are COPII vesiculation events at the surface of the ER and in higher plants are only visualizable in the electron microscope through cryofixation techniques. Fluorescent COPII labeling moves with Golgi stacks and locates to the interface between the ER and the Golgi. In contrast, the domain of the ER where retrograde COPI vesicles fuse, i.e., ER import sites (ERIS), has remained unclear. To identify ERIS we have employed ER-located SNAREs and tethering factors. We screened several SNAREs (SYP81, the SYP7 family, and USE1) to find a SNARE whose overexpression did not disrupt ER-Golgi traffic and which gave rise to discrete fluorescent punctae when expressed with an XFP tag. Only the Qc-SNARE SYP72 fulfilled these criteria. When coexpressed with SYP72-YFP, both the type I-membrane protein RFP-p24δ5 and the luminal marker CFP-HDEL whose ER localization are due to an efficient COPI-mediated recycling, form nodules along the tubular ER network. SYP72-YFP colocalizes with these nodules which are not seen when RFP-p24δ5 or CFP-HDEL is expressed alone or when SYP72-YFP is coexpressed with a mutant form of RFP-p24δ5 that cannot exit the ER. SYP72-YFP does not colocalize with Golgi markers, except when the Golgi stacks are immobilized through actin depolymerization. Endogenous SYP7 SNAREs, also colocalize with immobilized COPII/Golgi. In contrast, XFP-tagged versions of plant homologs to TIP20 of the Dsl1 COPI-tethering factor complex, and the COPII-tethering factor p115 colocalize perfectly with Golgi stacks irrespective of the motile status. These data suggest that COPI vesicle fusion with the ER is restricted to periods when Golgi stacks are stationary, but that when moving both COPII and COPI vesicles are tethered and collect in the ER-Golgi interface. Thus, the Golgi stack and an associated domain of the ER thereby constitute a mobile secretory and recycling unit: a unique feature in eukaryotic cells.